Process and apparatus for heat-treating substrate having film-forming composition thereon

ABSTRACT

Firing process and apparatus for uniformly heat-treating a substrate having a film-forming composition thereon, wherein the substrate is subjected to a first soaking step in which the substrate is held for a predetermined time in a first heating chamber whose temperature is maintained at a first value, so that the temperature within the substrate is held at the first value evenly throughout an entire mass of the substrate, and after feeding of the substrate into a second heating chamber whose temperature is maintained at a predetermined second value which is different from the first value by a predetermined difference, the substrate is subjected to a second soaking step in which the substrate is held for a second predetermined time in the second heating chamber, so that the temperature within the substrate is held at the second value evenly throughout the entire mass of the substrate.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a process and an apparatus foruniformly heat-treating a substrate which has a film-forming compositionthereon.

[0003] 2. Discussion of the Related Art

[0004] There are known substrates having films or layers formed thereonof a given material such as a metallic or inorganic material. In thepresent specification, the term “film” and the term “layer” are usedinterchangeably, unless otherwise specified. Such substrates includeglass substrates made of a glass material, typically, a soda-lime glass,and ceramic substrates made of a ceramic material, typically, alumina. Afilm or layer, which has a certain function, may be bonded to thesubstrate by fusion or melting of a glass bonding component or bysoftening, melting or sintering of the material per se. These substratesmay be used for anode plates for vacuum fluorescent displays (VFD),plasma switching boards for plasma address liquid crystal displays(PALC), field-emission displays (FED) and other display devices,thick-film wiring boards, and various electronic devices such as thermalprinter heads and image sensors. Generally, the substrates for theseelectronic devices are subjected to heat treatments at temperatures ofabout 500-650° C. for the purpose of annealing the substrates per se orforming functional films with a glass material used as a bonding agent.Where the substrates are ceramic, the substrates are heat-treated atabout 500-900° C. for forming functional films with a glass materialused as a bonding agent or for forming functional films of a metallicmaterial by utilizing the fusion of the metallic material at theinterface with the substrates.

[0005] Recently, there have been increasing requirements for increasingthe number of conductive, resistive, dielectric and other layers orfilms formed in desired patterns, and for increasing the density of suchlayers or films. Further, there has been an increasing demand fordisplay devices having a large-sized display screen, and an accordinglyincreasing requirement for increasing the size of the substrates forsuch large-sized display devices. To meet these requirements, it isrequired to form minutely patterned layers or films over a comparativelylarge area, particularly, on the substrates for the display devices. Thesubstrates for electronic devices described above have patternedfunctional films having minute cells or cavities. To assure highdimensional and positional accuracy of these minute cells, thefunctional films should be patterned with an improved degree ofuniformity. However, the above heat treatment or firing of thesubstrates has an influence on the quality of the substrates, whichinfluence increases as the size of the substrates increases. Therefore,the heat treatment causes a variation in the quality of the productsusing the substrates, and provides some restrictions in the design ofthe products, or reduces the yield of the products. The qualityvariation may be a variation in the resistance value of a resistor film,a variation in the withstand voltage of a dielectric film, a variationin the thickness due to uneven ratio of removal of binders by firing ofthe dielectric film, a variation in the continuity or resistance of aconductive film, a variation in the ease of wire-bonding or sputteringon the conductive film. Where the substrate suffers from a dimensionalchange due to expansion or shrinkage of its material upon heattreatment, it is difficult to accurately position the patternedfunctional films relative to each other, since each function film isfired after it is formed in a predetermined pattern. The uniformity andpositioning accuracy of the patterned films tend to be deteriorated withan increase in the density (minuteness) and size of the substrate,whereby the yield ratio of the product is significantly lowered as thedensity or size of the substrate is increased. In the case of asubstrate for a plasma display device having a screen size as large as40 inches, for example, the causes for lowering the yield ratio mayinclude: insufficient dimensional accuracy of multiple layers which formmultiple cells; variation in the height and width dimensions ofpartition walls; variation in the resistance of resistor cells;variation in the withstand voltage of a dielectric layer; an overalldimensional variation; and inaccurate positioning of front and rearplates which form a discharge cell.

SUMMARY OF THE INVENTION

[0006] The present invention was made in the light of the prior artdrawbacks described above. It is therefore a first object of thisinvention to provide a process of firing a substrate with a film-formingcomposition provided thereon, which permits uniform heating of thesubstrate to thereby assure a high yield ratio of a product includingthe substrate.

[0007] It is a second object of the invention to provide a firingapparatus suitable for practicing the method indicated above.

[0008] The first object may be achieved according to a first aspect ofthe present invention, which provides a firing process of uniformlyheat-treating a substrate having a film-forming composition thereon,comprising the steps of: (a) a first soaking step of holding, for apredetermined first time, said substrate in a first heating chamberwhose temperature is maintained at a predetermined first value, so thatthe temperature within said substrate is held at said first value evenlythroughout an entire mass of said substrate; (b) a feeding step offeeding said substrate subjected to said first soaking step, to a secondheating chamber whose temperature is maintained at a predeterminedsecond value which is different from said predetermined first value by apredetermined difference; and (c) a second soaking step of holding saidsubstrate in said second heating chamber for a predetermined secondtime, so that the temperature within said substrate is held at saidsecond value evenly throughout the entire mass of said substrate.

[0009] In the present firing process, the substrate having afilm-forming composition is heat-treated by first subjecting thesubstrate to the first soaking step in the first heating chamber inwhich the substrate is held at the predetermined first temperature valuefor the predetermined first time for even distribution of thetemperature throughout the entire mass of the substrate. Then, thesubstrate is fed into the second heating chamber whose temperature ismaintained at the predetermined second value different from the firstvalue by the predetermined amount, and is subjected to the secondsoaking step in which the substrate is held at the second value for thepredetermined second time for even distribution of the temperaturethroughout the entire mass of the substrate. The substrate may besubjected to a further soaking step or steps. Thus, the substrate isheat-treated at different temperatures which are different from eachother, so that a local variation in the temperature within the substrateand the film-forming composition is minimized. Where the substrate isformed of a glass material and is heat-treated at temperatures higherthan the strain point of the glass material, a local dimensionalvariation or configurational deviation of the substrate can beminimized. Accordingly, the present firing process permits accuratepositioning accuracy of films, layers or any other structural featuressubsequently formed on the substrate, resulting in a considerablyincreased yield ratio of the product which includes the substrate, evenwhere the substrate has minute or intricate structural patterns or has arelatively large size. The film-forming composition provided on thesurface of the substrate may be thick-film dielectric films, dielectricpartition walls, thick-film resistor films, electrode films or inorganicpigment films. Since the present firing process makes it possible tominimize the local temperature variation within the substrate and withinsuch films or layers formed thereon, a glass material contained in thefilms or layers as a bonding component may be uniformly melted orsoftened in the heat treatment process, and a metallic material or metaloxide contained in the films may be uniformly melted or sintered.Accordingly, the product has reduced variation in its properties such aswithstand voltage, resistance value, discharge characteristics andoptical filter characteristics and in its dimensions such as height andwidth dimensions of the partition walls. Consequently, the yield ratioof the product is significantly improved, even where the substrate has alarge size. Further, the reduced variation in the resistance valueresults in a reduced cost of control of the production steps andelimination of some steps such as trimming step.

[0010] In the present process, the first and second temperature valuesare preferably determined to be close to the transition or strain pointof a glass material contained in the substrate so that the temperatureof the substrate changes through the transition or strain point whilethe temperature within the substrate is evenly distributed throughoutthe entire mass of the substrate. Where the films are bonded to thesubstrate by melting or sintering of a metallic or inorganic material,the first and second temperature values are preferably determined to beclose to the melting or sintering point of the metallic or inorganicmaterial so that the temperature of the substrate changes through themelting or sintering point while the temperature within the substrate isevenly distributed throughout the entire mass of the substrate.

[0011] In one preferred form of the first aspect of the invention, thefiring process further comprises a first stand-by step which isimplemented concurrently with the first soaking step, to adjust thetemperature in the second heating chamber to the predetermined secondvalue so that the second soaking step is implemented in the secondheating chamber, and further comprising a second stand-by step which isimplemented concurrently with the second soaking step, to adjust thetemperature in the first heating chamber to a predetermined third valuewhich is different from the predetermined second value by apredetermined difference, so that a third soaking step is implemented inthe first heating chamber. In this case, the temperature in the secondheating chamber is maintained at the second value while the firstsoaking step is implemented, and the temperature in the first heatingchamber is maintained at the third value while the second soaking stepis implemented. Accordingly, the second and third soaking steps may beinitiated immediately after the termination of the first and secondsoaking steps, respectively.

[0012] In one advantageous arrangement of the above first preferred formof the firing process, the first stand-by step comprises a cooling stepof lowering the temperature in the second heating chamber to a valuelower than the predetermined second value by a predetermined amount, anda temperature raising and holding step of effecting feed-back control toraise the temperature in the second heating chamber to the predeterminedsecond value and maintain the second value, and wherein the secondstand-by step comprises a cooling step of lowering the temperature inthe first heating chamber to a value lower than the predetermined thirdvalue by a predetermined amount, and a temperature raising and holdingstep of effecting feed-back control to raise the temperature in thefirst heating chamber to the predetermined third value and maintain thethird value. In this arrangement, the temperature in the second heatingchamber is first lowered below the second value and then raised to thesecond value while the first soaking step is implemented in the firstheating chamber. Similarly, the temperature in the first heating chamberis first lowered below the third value and then raised to the thirdvalue while the second soaking step is implemented in the second heatingchamber. This arrangement permits the second and third temperaturevalues to be rapidly established in the first and second stand-by steps.

[0013] The second object indicated above may be achieved according to asecond aspect of this invention, which provides a firing apparatus foruniformly heat-treating a substrate having a film-forming compositionthereon, comprising: (a) shutter devices which partially define a firstand a second heating chamber such that said first and second heatingchambers are thermally insulated from each other; (b) a heating devicefor controlling temperatures in said first and second heating chambers,independently of each other; (c) a feeding device for feeding saidsubstrate into said first and second heating chambers alternately, sothat said substrate is heat-treated in said first and second chambersalternately; and (d) a control device for controlling said heatingdevice to maintain the temperature in said first heating chamber at apredetermined first value while said substrate is heat-treated in saidfirst heating chamber, and adjusting the temperature in said secondheating chamber to a predetermined second value different from saidpredetermined first value by a predetermined difference while saidsubstrate is heat-treated in said first heating chamber, said controldevice adjusting the temperature in said first heating chamber to apredetermined third value different from said predetermined second valueby a predetermined difference while said substrate is heat-treated insaid second heating chamber, so that said substrate is then heat-treatedin said first heating chamber at said third value.

[0014] In the present firing apparatus, the substrate having afilm-forming composition is heat-treated in the first heating chamber atthe predetermined first temperature value for the predetermined firsttime for even distribution of the temperature throughout the entire massof the substrate. Then, the substrate is fed by the feeding device intothe second heating chamber the temperature of which is maintained at thepredetermined second value different from the first value, and isheat-treated in the second heating chamber at the second value for thepredetermined second time for even distribution of the temperaturethroughout the entire mass of the substrate. The temperature in thefirst heating chamber is adjusted to the predetermined third valuedifferent from the second value while the substrate is heat-treated inthe second heating chamber, and the substrate is then heat-treated inthe first heating chamber at the third value. Thus, the substrate isalternately placed in the first and second heating chambers, andheat-treated there at different temperatures, so that a local variationin the temperature within the substrate and the film-forming compositionis minimized. Where the substrate is formed of a glass material and isheat-treated at temperatures higher than the strain point of the glassmaterial, a local dimensional variation or configurational deviation ofthe substrate can be minimized. Accordingly, the present firingapparatus permits accurate positioning accuracy of films, layers or anyother structural features subsequently formed on the substrate,resulting in a considerably increased yield ratio of the product whichincludes the substrate, even where the substrate has minute or intricatestructural patterns or has a relatively large size. The film-formingcomposition provided on the surface of the substrate may be thick-filmdielectric films, dielectric partition walls, thick-film resistor films,electrode films or inorganic pigment films. Since the present firingapparatus makes it possible to minimize the local temperature variationwithin the substrate and within such films or layers formed thereon, aglass material contained in the films or layers as a bonding componentmay be uniformly melted or softened in the heat treatment process, and ametallic material or metal oxide contained in the films may be uniformlymelted or sintered. Accordingly, the product has reduced variation inits properties such as withstand voltage, resistance value, dischargecharacteristics and optical filter characteristics and in its dimensionssuch as height and width dimensions of the partition walls.Consequently, the yield ratio of the product is significantly improved,even where the substrate has a large size. Further, the reducedvariation in the resistance value results in a reduced cost of controlof the production steps and elimination of some steps such as trimmingstep.

[0015] Further, the present firing apparatus uses only the two heatingchambers for heat treatment of the substrate, namely, only the first andsecond heating chambers to which the substrate is alternately fed forheat treatment. Accordingly, the longitudinal dimension of the firingapparatus is advantageously reduced.

[0016] In the present firing apparatus, the first, second and thirdtemperature values are preferably determined to be close to thetransition or strain point of a glass material or the melting orsintering point of a metallic or inorganic material so that thetemperature of the substrate changes through the transition, strain,melting or sintering point indicated above, while the temperature of thesubstrate is evenly distributed throughout the substrate, as describedabove with respect to the firing process.

[0017] In one preferred form of the firing apparatus according to thesecond aspect of this invention, the apparatus further comprises acooling device for lowering the temperatures in said first and secondheating chambers. In this case, the temperature in the stand-by heatingchamber in which the substrate is not currently heat-treated may beadjusted to the predetermined value, by first operating the coolingdevice to positively lower the temperature in the stand-by heatingchamber to a level lower than the predetermined value and thencontrolling the heating device to adjust the temperature in the stand-byheating chamber to the predetermined value. The initial operation of thecooling device and the subsequent operation of the heating device permitrapid and uniform adjustment of the temperature in the stand-by heatingchamber to the predetermined value at which the substrate is thenheat-treated.

[0018] Preferably, the cooling device comprises cooling tubes fordelivering cooling air into the first and second heating chambers, sothat the temperature in the stand-by heating chamber is lowered by thecooling air delivered by the cooling tubes.

[0019] In another preferred form of the above firing apparatus, each ofthe shutter devices includes a shutter member movable between an openposition and a closed position for thermal insulation of the first andsecond heating chambers. The shutter member is placed in the openposition when the substrate is fed by the feeding device into or fromthe first or second heating chamber, and in the closed position whilethe substrate is heat-treated in one of the first and second heatingchambers and while the temperature in the other heating chamber isadjusted. Since the shutter members of the shutter devices assurethermal insulation of the first and second heating chambers, thetemperature can be evenly or uniformly distributed within the heatingchambers during heat treatment of the substrate or during adjustment ofthe temperature in the stand-by heating chamber. Accordingly, thepresent arrangement permits a further reduced local variation in thetemperature within the substrate.

[0020] The second object indicated above may also be achieved accordingto a third aspect of this invention, which provides a firing apparatusfor uniformly heat-treating a substrate having a film-formingcomposition thereon, comprising: (a) shutter devices which partiallydefine at least two heating chambers including a first and a secondheating chamber such that said at least two heating chambers arethermally insulated from each other; (b) a heating device forcontrolling temperatures in said first and second heating chambersindependently of each other; (c) a temperature control device forcontrolling said heating device to maintain the temperature in saidfirst heating chamber at a predetermined first value uniformlythroughout said first heating chamber, and to maintain the temperaturein said second heating chamber at a predetermined second value uniformlythroughout said second heating chamber, said second value beingdifferent from said first value by a predetermined difference; and (d) afeeding device for feeding said substrate in one feeding direction,first into said first heating chamber for heat-treating said substrateat said predetermined first value for a predetermined first time, andthen into said second heating chamber for heat-treating said substrateat said predetermined second value for a predetermined second time, saidfeeding device further feeding said substrate from said second heatingchamber after heat treatment thereof in said second heating chamber.

[0021] In the present firing apparatus according to the third aspect ofthe invention, the substrate having a film-forming composition isheat-treated in the first heating chamber at the predetermined firsttemperature value for the predetermined first time for even distributionof the temperature throughout the entire mass of the substrate. Then,the substrate is fed by the feeding device into the second heatingchamber the temperature of which is maintained at the predeterminedsecond value different from the first value, and is heat-treated in thesecond heating chamber at the second value for the predetermined secondtime for even distribution of the temperature throughout the entire massof the substrate. Thus, the substrate is heat-treated in the first andsecond heating chambers at different temperatures so that a localvariation in the temperature within the substrate and the film-formingcomposition is minimized. Where the substrate is formed of a glassmaterial and is heat-treated at temperatures higher than the strainpoint of the glass material, a local dimensional variation orconfigurational deviation of the substrate can be minimized.Accordingly, the present firing apparatus permits accurate positioningaccuracy of films, layers or any other structural features subsequentlyformed on the substrate, resulting in a considerably increased yieldratio of the product which includes the substrate, even where thesubstrate has minute or intricate structural patterns or has arelatively large size. The film-forming composition provided on thesurface of the substrate may be thick-film dielectric films, dielectricpartition walls, thick-film resistor films, electrode films or inorganicpigment films. Since the present firing apparatus makes it possible tominimize the local temperature variation within the substrate and withinsuch films or layers formed thereon, a glass material contained in thefilms or layers as a bonding component may be uniformly melted orsoftened in the heat treatment process, and a metallic material or metaloxide contained in the films may be uniformly melted or sintered.Accordingly, the product has reduced variation in its properties such aswithstand voltage, resistance value, discharge characteristics andoptical filter characteristics and in its dimensions such as height andwidth dimensions of the partition walls. Consequently, the yield ratioof the product is significantly improved, even where the substrate has alarge size. Further, the reduced variation in the resistance valueresults in a reduced cost of control of the production steps andelimination of some steps such as trimming step.

[0022] Further, the substrate is heat-treated at the different first andsecond temperature values in the first and second heating chambers whileit is fed in one direction by the feeding device. Accordingly, theoverall length of the present firing apparatus can be made smaller thanthat of a conventional continuous feeding type firing apparatus which isadapted to continuously feed the substrate so as to cool the temperatureof the substrate according to a continuous temperature cooling patternthat permits the substrate to have an extremely reduced localtemperature variation. Since the present apparatus does not have astand-by heating chamber as provided in a shutter type apparatus inwhich the substrate is reciprocated between two heating chambers, thepresent apparatus provides an accordingly increased degree of heattreating efficiency and is suitable for mass production of a productusing the substrate.

[0023] In one preferred form of the present apparatus according to thethird aspect of the invention, each of the shutter devices includes ashutter member movable between an open position and a closed positionfor thermal insulation of the first and second heating chambers. Themovable shutter member is placed in the open position when the substrateis fed by the feeding device into or from the first or second heatingchamber, and in the closed position while the substrate is heat-treatedin the first or second heating chamber. Since the shutter members of theshutter devices assure thermal insulation of the first and secondheating chambers, the temperature can be evenly or uniformly distributedwithin the heating chambers during heat treatment of the substrate.Accordingly, the present arrangement permits a further reduced localvariation in the temperature within the substrate.

[0024] In another preferred form of the present apparatus, the feedingdevice comprises a plurality of rollers whose axes of rotation areparallel to each other and perpendicular to the above-indicated onefeeding direction and which are arranged in this feeding direction tosupport the substrate. The rollers are rotated to feed the substrate inthe feeding direction. In this arrangement, the substrate is supportedby the plurality of rollers and fed in the predetermined feedingdirection with the rollers being rotated. Thus, the rollers are used inplace of a generally used endless belt made of a mesh of refractorymetal, for example. In the present arrangement, the two or more heatingchambers including the first and second heating chambers and the feedingdevice provide a roller hearth kiln for firing the substrate havingfilms formed thereon. In this roller hearth kiln, the films formed onthe substrate are less likely to be adversely influenced by dust whichmay be considerably scattered in a heating area using a conveyor belt.Namely, the feeding of the substrate by the rotating rollers is lesslikely to deteriorate the function of the films on the substrate due todust during the heat treatment therein.

[0025] In one advantageous arrangement of the above preferred form ofthe apparatus, one of the shutter devices includes a shutter which ismovable in a vertical direction perpendicular to the feeding direction,between an open position and a closed position, through a gap betweenadjacent ones of the plurality of rollers. The shutter placed in theclosed position separates the first and second heating chambers fromeach other with thermal insulation therebetween. In this arrangement,the shutter is vertically movable without an interference with therollers, permitting complete thermal insulation of the first and secondheating chambers, and assuring improved uniformity of temperature ineach heating chamber and accordingly reduced local variation of thetemperature within the substrate.

[0026] In another advantageous arrangement of the above preferred formof the apparatus, each of the plurality of rollers is made of a ceramicmaterial. In this case, the rollers are less likely to be worn, rusted,damaged or deteriorated due to contact with the substrate and heating inthe heating chambers, assuring an reduced amount of dust produced in theheating chambers and accordingly enhanced quality of the firedsubstrate.

[0027] Where the rollers are made of a ceramic material, each of theabove-indicated two heating chambers has an inner wall surfacepreferably made of a ceramic material, and the shutter of each shutterdevice is also preferably made of a ceramic material. Thus, the rollersof the feeding device, the shutters and the inner wall surfaces of theheating chambers are all made of the ceramic material, and are lesslikely to be worn, rusted, damaged or deteriorated due to heating,assuring a further reduced amount of dust produced in the heatingchambers.

[0028] In a further preferred form of the firing apparatus according tothe third aspect of the invention, the feeding device includes anintermittently feeding device for intermittently feeding the substrateby rotation of the plurality of rollers through the at least two heatingchambers, and a continuously feeding device for continuously feeding thesubstrate by rotation of the rollers at a predetermined feeding speedthrough a continuous heat treatment zone which includes an area adjacentto the at least two heating chambers. The continuously feeding deviceincludes a feeding speed changing device for changing rotating speeds ofthe rollers in the above-indicated area so that a feeding speed of thesubstrate in this area is almost equal to the feeding speed by theintermittently feeding device.

[0029] In the above preferred form of the firing apparatus, thesubstrate is fed intermittently through the at least two heatingchambers, and is continuously fed at a given speed through thecontinuous heat treatment zone. In the above-indicated area of thecontinuous heat treatment area adjacent to the first heating chamber,for example, the rotating speed of the rollers and the feeding speed ofthe substrate are raised to those of the intermittently feeding device,so that the substrate may be smoothly and relatively rapidly fed fromthe above area into the first heating chamber, so that the time requiredfor the substrate to move between the above area and the first heatingchamber is shortened, making it possible to reduce the local variationof the temperature within the substrate due to a difference in thetemperatures between the above area of the continuous heat treatmentzone and the first heating chamber, for example. Further, the feedingspeed changing device is effective to reduce an amount of slidingmovement between the rollers and the substrate due to the difference inthe feeding speed between the above area and the first heating chamber,whereby the amount of dust produced in the furnace is accordinglyreduced. It is also noted that the shutters of the shutter devices areplaced in their open position for a time as short as possible, so as tominimize a deviation of the temperature in each heating chamber from thepredetermined values and an uneven temperature distribution within eachheating chamber.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The above and optional objects, features, advantages andtechnical and industrial significance of this invention will be betterunderstood by reading the following detailed description of presentlypreferred embodiments of the invention, when considered in connectionwith the accompanying drawings, in which:

[0031]FIG. 1 is a front elevational view of a two-chamber type firingapparatus constructed according to one embodiment of this invention,with a furnace being shown in cross section;

[0032]FIG. 2 is a plan view of the firing apparatus of FIG. 1, with anadiabatic wall being shown in cross section;

[0033]FIG. 3 is an end elevational view of the firing apparatus of FIG.1, taken along line 3-3 of FIG. 1;

[0034]FIG. 4 is a perspective view of a cooling tube used in theembodiment of FIG. 1;

[0035]FIG. 5 is a view showing an arrangement of a plurality of heatersin the embodiment of FIG. 1;

[0036]FIG. 6 is a block diagram illustrating a control system of thefiring apparatus of FIG. 1;

[0037]FIG. 7 is a flow chart illustrating an operation of an arithmeticcontrol circuit in the control system of FIG. 6;

[0038]FIG. 8 is a time chart indicating a change in the temperature of asubstrate controlled by the operation illustrated in the flow chart ofFIG. 7;

[0039]FIG. 9 is a view for explaining local deformation of the substratein a conventional firing apparatus;

[0040]FIG. 10 is a schematic view showing an arrangement of a continuoustype firing apparatus constructed according to another embodiment ofthis invention;

[0041]FIG. 11 is a view showing an overall arrangement of a continuoustype firing apparatus constructed according to a further embodiment ofthis invention;

[0042] FIGS. 12(a)-12(c) are views showing conveyor devices used in theembodiment of FIG. 11;

[0043]FIG. 13 is a view partly in longitudinal cross section of thefurnace in the embodiment of FIG. 11; and

[0044] FIGS. 14(a), 14(b), 14(c), 14(d) and 14(e) are cross sectionalviews taken along lines a-a, b-b, c-c, d-d and e-e of FIG. 13,respectively.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0045] Referring first to the front elevational view of FIG. 1, planview of FIG. 2 and side elevational view of FIG. 3, there is shown ashuttle type or two-chamber type firing apparatus 10 constructedaccording to one embodiment of this invention.

[0046] The firing apparatus 10 includes a generally planar tunnel typemuffle furnace 12 made of stainless steel SUS310 or Inconel (Ni-Cr-Fealloy). The muffle furnace 12 is accommodated in an adiabatic wallstructure 14, except at its longitudinal opposite end portions. Theadiabatic wall structure 14 is supported by a frame 16, and the mufflefurnace 12 is supported in a horizontal posture by the adiabatic wallstructure 14. An endless belt 18 formed from a stainless or Inconel meshis run within the frame 16 such that an upper space of the endless belt18 extends through the tunnel type muffle furnace 12. This endless belt18 is driven by a belt drive device 20 disposed in the frame 16.

[0047] The muffle furnace 12 is provided with an inlet shutter device22, an intermediate shutter device 24 and an outlet shutter device 26 atits inlet end portion, intermediate portion and outlet end portion,respectively. With these shutter devices 22, 24, 26 placed in theirclosed positions, the interior of the muffle furnace 12 is divided intoa first heating chamber 28 and a second heating chamber 30 which arethermally insulated from each other, so that the temperatures withinthese two heating chambers 28, 30 can be controlled independently ofeach other. The shutter devices 22, 24, 26 include respective shuttermembers 38, 40, 42 which are vertically movable along respective shutterpaths 32, 34, 36 between the lower closed position and the upper openposition. The shutter devices 22, 24, 26 further include respectiveshutter drive motors 44, 46, 48 for vertically moving the shuttermembers 38, 40, 42, respectively.

[0048] Between the muffle furnace 12 and the adiabatic wall structure14, there is formed a space which is divided by a partition wall 50 andthe shutter path 34 into a first vacuum chamber 52 and a second vacuumchamber 54, which surround the first and second heating chambers 28, 30of the muffle furnace 12, respectively. A plurality of air dischargerpassages 56 are formed through the upper wall of the adiabatic wallstructure 14, for evacuating the first and second vacuum chambers 52,54. The air discharger passages 56 for the first vacuum chamber 52 arenot shown in FIGS. 1 and 2. Each air discharger passage 56 is providedwith a suitable damper. A plurality of cooling tubes 58 each having aplurality of nozzles 57 as shown in FIG. 4 are provided in each of thefirst and second vacuum chambers 52, 54, for delivering cooling airagainst the muffle furnace 12 to positively lower the temperatures inthe first and second heating chambers 28, 30. As described below, thefirst and second heating chambers 28, 30 are alternately used to processa substrate 62 such that the substrate 62 is heat-treated at a giventemperature in one of these two heating chambers 28, 30 while thetemperature in the other heating chamber (hereinafter referred to as“stand-by heating chamber”) is controlled to a level different from thetemperature in the above-indicated one heating chamber (hereinafterreferred to as “operating heating chamber”), so that the substrate 62 isthen introduced into the stand-by heating chamber for the heat treatmentthereof. Before the temperature in the stand-by heating chamber 28, 30is controlled to the predetermined level, the cooling air is deliveredby the cooling tubes 58 against the portion of the muffle furnace 12corresponding to the stand-by heating chamber, for temporarilypositively lower the temperature in the stand-by heating chamber. Thispre-cooling of the stand-by heating chamber permits its temperature tobe subsequently controlled rapidly and uniformly to the predeterminedlevel.

[0049] At each of the inlet and outlet end portions of the mufflefurnace 12, there are provided three exhaust passages 60 each equippedwith a damper, for discharging exhaust gases from the first and secondheating chambers 28, 30.

[0050] On the inner surfaces of the upper and lower walls of theadiabatic wall structure 14 which partly define the first and secondvacuum chambers 52, 54, there are disposed a multiplicity of heaters Harranged in a matrix pattern. These heaters H consist of two or moregroups of heaters so that the heaters in one group are controlledindependently of the heaters in another group. For instance, the heatersH in the first vacuum chamber 52 consist of nine pairs of heaters H₁₁₁,H₁₁₂, H₁₁₃, H₁₂₁, H₁₂₂, H₁₂₃, H₁₃₁, H₁₃₂, H₁₃₃ arranged in a matrix ofthree columns extending in the feeding direction of the substrate 62 bythe endless belt 18 and three rows extending in the width direction ofthe endless belt 18, as shown in FIG. 5. Each pair of heaters consistsof an upper heater and a lower heater provided on the upper and lowerwalls of the adiabatic wall structure 14. Similarly, the heaters H inthe second vacuum chamber 54 consist of nine pairs of heaters H₂₁₁,H₂₁₂, H₂₁₃, H₂₂₁, H₂₂₂, H₂₂₃, H₂₃₁, H² ₃₂, H₂₃₃ arranged in a matrix ofthree columns and three rows.

[0051] A multiplicity of temperature detectors T are provided atrespective positions corresponding to those of the heaters H, fordetecting the temperature at the corresponding positions in the firstand second heating chambers 28, 30. That is, the heaters T consist ofnine temperature detectors T₁₁₁, T₁₁₂, T₁₁₃, T₁₂₁, T₁₂₂, T₁₂₃, T₁₃₁,T₁₃₂, T₁₃₃ for detecting the heating temperatures of the nine pairs ofheaters H₁₁₁, H₁₁₂, H₁₁₃, H₁₂₁, H₁₂₂, H₁₂₃, H₁₃₁, H₁₃₂, H₁₃₃ in thefirst heating chamber 28, and nine temperature detectors T₂₁₁, T₂₁₂,T₂₁₃, T₂₂₁, T₂₂₂, T₂₂₃, T₂₃₁, T₂₃₂, T₂₃₃ for detecting the heatingtemperatures of the nine pairs of heaters H₂₁₁, H₂₁₂, H₂₁₃, H₂₂₁, H₂₂₂,H₂₂₃, H₂₃₁, H₂₃₂, H₂₃₃ in the second heating chamber 30, as indicated inFIG. 6. In FIGS. 1-3, only some of these heaters T are shown.

[0052] The firing apparatus 10 further includes a control console 64 asshown in FIG. 1, which incorporates a control device 66 as shown in FIG.6. The control device 66 includes a multiplexer 68 for combining in atime-sharing manner the output signals of the temperature detectorsT₁₁₁, T₁₁₂, T₁₁₃, T₁₂₁, T₁₂₂, T₁₂₃, T₁₃₁, T₁₃₂, T₁₃₃ for detecting thetemperatures in the first heating chamber 28 and the temperaturedetectors T₂₁₁,T₂₁₂, T₂₁₃, T₂₂₁, T₂₂₂, T₂₂₃, T₂₃₁, T₂₃₂, T₂₃₃ fordetecting the temperatures in the second heating chamber 30. The outputsignals of these temperature detectors T are fed from the multiplexer 68to an A/D convertor 70, and digital output signals of the A/D convertor70 are fed to an arithmetic control circuit 72. For example, thisarithmetic control circuit 72 is constituted by a microcomputer which isadapted to process the output of the A/D convertor 70 according tocontrol programs stored in a read-only memory while utilizing atemporary data storage function of a random-access memory. Thearithmetic control circuit 72 generates control signals to be appliedthrough an output interface 74 to a motor driver MD₁, heater driversD₁₁₁, D₁₁₂, D₁₁₃, D₁₂₁, D₁₂₂, D₁₂₃, D₁₃₁, D₁₃₂, D₁₃₃, D₂₁₁, D₂₁₂, D₂₁₃,D₂₂₁, D₂₂₂, D₂₂₃, D₂₃₁, D₂₃₂, D₂₃₃ and motor drivers MD₂, MD₃ and MD₄.The motor driver MD₁ is provided for activating the belt drive device20, and the heater drivers MD are provided for activating the heatersH₂₁₁, H₂₁₂, H₂₁₃, H₂₂₁, H₂₂₂, H₂₂₃, H₂₃₁, H₂₃₂, H₂₃₃. The motor driversMD₂, MD₃ and MD₄ are provided for activating the shutter drive motors44, 46, 48, respectively.

[0053] The above-indicated heaters H₂₁₁, H₂₁₂, H₂₁₃, H₂₂₁, H₂₂₂, H₂₂₃,H₂₃₁, H₂₃₂, H₂₃₃ are controlled according to target temperature valuesat the corresponding positions or according to relative output ratios ofthose heaters, so that the temperature within each of the first andsecond heating chambers 28, 30 is held even throughout the space in theheating chamber. For instance, the outputs of the heaters H₁₁₁, H₁₁₃,H₁₂₁, H₁₂₃, H₁₃₁, H₁₃₃, H₂₁₁, H₂₁₃, H₂₂₁, H₂₂₃, H₂₃₁, H₂₃₃ in the outercolumns located at the widthwise ends of the muffle furnace 12 are madehigher than those of the heaters H₁₁₂, H₁₂₂, H₁₃₂, H₂₁₂, H₂₂₂, H₂₃₂ inthe intermediate column located at the widthwise central position of themuffle furnace 12. Similarly, the outputs of the heaters H₁₃₁, H₁₃₂,H₁₃₃, H₂₃₁, H₂₃₂, H₂₃₃ in the outermost rows located at thelongitudinally ends of the muffle furnace 12 are made higher than thoseof the heaters H₁₂₁, H₁₂₂, H₁₂₃, H₂₂₁, H₂₂₂, H₂₂₃ in the inner rowslocated at the longitudinally inner positions of the muffle furnace 12.In this respect, it is noted that the heaters in the outer columns andin the outermost rows are more likely to be cooled than the otherheaters. Thus, the outputs of the heaters H₁₃₁, H₁₃₃, H₂₃₁, H₂₃₃ arecontrolled to be the highest, and the outputs of the heaters H₁₃₂, H₁₂₁,H₁₂₃, H₂₃₂, H₂₂₁, H₂₂₃ are controlled to be the second highest.

[0054] Referring next to the flow chart of FIG. 7, there will bedescribed an operation of the arithmetic control circuit 72 to controlthe firing apparatus 10. A routine illustrated in FIG. 7 is initiatedwith step S1 to determine whether a START signal for starting theoperation of the firing apparatus 10 is present as a result of operationof a START pushbutton provided on the control console 64. If anaffirmative decision (YES) is obtained in step S1, the control flow goesto step S2 in which the inlet shutter device 22 is operated to move theshutter member 38 to the upper open position, and the endless belt 18 isoperated so that the substrate 62 in a green or unfired state placed onthe endless belt 18 is loaded or introduced into the first heatingchamber 28. Then, the inlet shutter device 22 is operated to move theshutter member 38 to the lower closed position. This point of time isindicated at ti in Fig. B.

[0055] Step S2 is followed by step S3 in which the heaters T areactivated and feed-back controlled to raise the temperature in the firstheating chamber 28, according to a desired temperature raising pattern,during a time period A between the points of time t1 and t2 indicated inFIG. 8, wherein an example of the temperature raising pattern isindicated corresponding to the time period A. The time period A is atemperature raising period. Then, the control flow goes to step S4 todetermine whether the temperature in the first heating chamber 28 hasreached the highest firing temperature KT₁ (e.g., about 500° C.) of thetemperature raising pattern. If a negative decision (NO) is obtained instep S4, steps S3 and S4 are repeatedly implemented until an affirmativedecision (YES) is obtained in step S4, that is, until the highesttemperature KT₁ is reached. If the affirmative decision (YES) isobtained in step S4, the control flow goes to step S5 and the followingsteps to initiate a cooling operation. That is, a cooling period beginsat the point of time t2 indicated in FIG. 8.

[0056] It is noted that the condition in which the cooling step isperformed after the highest temperature KT₁ (e.g., about 500° C. orhigher) of the temperature raising pattern is reached is important inthe heat treatment of the substrate 62. For instance, the substrate 62is used for a vacuum fluorescent display (VFD), a plasma display panel(PDP), a plasma address liquid crystal display (PALC) or afield-emission display (FED). The substrate 62 for such devices may beof a glass such as soda-lime glass having a relatively low strain point.In this case, the temperature within the substrate 62 is likely to beuneven and have different cooling rates at different local portionsthereof, leading to a local dimensional variation or deviation thereof,which makes it difficult to achieve accurate alignment or relativepositioning of multiple thick films formed by printing thereon. Thelocal dimensional variation or deviation may also make it impossible toaccurately align printed thick films of front and rear plates which formmultiple cells in a plasma display panel (PDP) or field-emission display(FED). The yield ratio of the display device, which is lowered due tosuch local dimensional variation or deviation, is lowered with anincrease in the screen size of the display device, which means anincrease in the temperature variation of the substrate 62. When thescreen size is as large as 40 inches, for instance, the yield ratio ofthe display device is considerably low. FIG. 9 shows an example of thelocal dimension variation of the substrate 62 caused by differentcooling rates at different positions according to a conventional firingprocess. Described more specifically, the substrate 62 heated and cooledin the conventional process has a smaller width dimension at itstrailing end than at its leading end as viewed in the feeding direction,as indicated by solid line, when the cooling rate is higher at theleading end than at the trailing end. In FIG. 9, one-dot chain lineindicates the dimension prior to the firing of the substrate 62. Wherethe substrate 62 has a multiplicity of thick-film resistors or bondingpads formed thereon by printing, the resistance value of the resistorsand the ease of bonding of the bonding pads are influenced by differentcooling rates at different positions of the substrate, which are causedby uneven distribution of the temperature within the substrate, sincethe different cooling rates cause a variation in the fusion, melting orsoftening of a glass component included as a bonding agent in thefunctional thick films, or cause a variation in the melting or sinteringof a powdered metal or inorganic material included in the thick films.The yield ratio of the product is lowered due to the variation in theresistance value of the resistors and the ease of bonding of the bondingpads, particularly, where the substrate is large. Where the substrate 62has rib walls formed by lamination of dielectric films or layers bythick-film printing, the temperature within the substrate 62 tends to beuneven, leading to a local variation in the cooling rate, which mayresults in a variation in the melting or softening of a glass componentcontained in the dielectric films, and a consequent variation in thefiring shrinkage of the substrate, namely, a variation in the thicknessand width dimensions of the dielectric films. In this respect, too, theyield ratio of the product is lowered with an increase in the size ofthe substrate 62.

[0057] Referring back to the flow chart of FIG. 7, step S4 is followedby step S5 if the affirmative decision (YES) is obtained in step S4 asmentioned above. In step S5, a counter N which is initially reset tozero is incremented. The counter N is incremented to select a targettemperature KT_(N) and a holding time HT in the following steps,according to predetermined data maps. Then, the control flow goes tostep S6 in which the heaters H in the first vacuum chamber 52 arefeed-back controlled to maintain the temperature in the first heatingchamber 28 (in which the substrate 62 is now accommodated) at a targettemperature KT_(N), namely, at a predetermined first target temperatureKT₁ which is the highest temperature according to the temperatureraising pattern, since the content of the counter N is now equal to “1”.Consequently, the temperature within the substrate 62 is maintained atthe first target value KT₁ with even temperature distribution throughoutthe substrate 62. Then, step S7 is implemented to feed-back control theheaters H in the second vacuum chamber 54 to maintain the temperature inthe second heating chamber 28 (in which the substrate 62 is notaccommodated) at a target temperature KT_(N+1), namely, at apredetermined second target temperature KT₂, since N+1 is now equal to2. The second target temperature KT₂ is lower than the first targettemperature KT₁ (highest temperature) by a predetermined decrement valueΔKT. This decrement value ΔKT may range from a few or several degrees ofcentigrade (° C.) to a few or several degrees higher than 10° C. In thefollowing description, one of the first and second heating chambers 28,30 in which the substrate 62 is now accommodated for heat treatment isreferred to as “operating heating chamber” while the other heatingchamber in which the substrate 62 is not accommodated is referred to as“stand-by heating chamber”, where appropriate. When step S5 isimplemented for the first time, the first heating chamber 28 is theoperating heating chamber while the second heating chamber 30 is thestand-by chamber which will accommodate the substrate 62 after the heattreatment thereof in the operating heating chamber.

[0058] Step S7 is followed by step S8 to determine whether the timeduring which the temperature in the first heating chamber 29 ismaintained at the first target temperature KT₁ has reached apredetermined holding time HT_(N), namely, a predetermined first holdingtime HT₁ since the content of the counter N is now equal to “1”. Thefirst holding time HT₁ may range from some minutes longer than 10minutes to a few or several tens of minutes. Initially, a negativedecision (NO) is obtained in step S8, and steps S6-S8 are repeatedlyimplemented until an affirmative decision (YES) is obtained in step S8.A time period corresponding to the first holding time HT₁ corresponds toa first soaking step (temperature equalizing step) for holding thetemperature in the first or operating heating chamber 28 at the firsttarget temperature KT₁ to establish even distribution of the temperaturethroughout an entire mass of the substrate 62, and also corresponds to astand-by step for adjust the temperature in the second or stand-byheating chamber 30 to the second target temperature KT₂, to therebyprepare the stand-by heating chamber 30 for a second soaking step(temperature equalizing step) for establishing even distribution of thetemperature throughout the entire mass of the substrate 62.

[0059] If the affirmative decision (YES) is obtained in step S8 duringrepeated implementation of steps S6-S8, the control flow goes to step S9(feeding step) in which the intermediate shutter device 24 is activatedto move the shutter member 40 to its upper open position, and theendless belt 18 is operated to feed the substrate from the first heatingchamber 28 into the second or stand-by chamber 30. The shutter member 40is then moved to its lower closed position. This point of time isindicated at t3 in FIG. 8.

[0060] Then, the control flow goes to step S10 to determine whether thecontent of the counter N is equal to “6”. When this step S10 isimplemented for the first time, a negative decision (NO) is obtained,and the control flow goes to step S5 to increment the counter N to “2”.Consequently, the heaters H in the second vacuum chamber 54 arefeed-back controlled in step S6 so that the temperature in the secondheating chamber 30 which is now the operating heating chamber ismaintained at the second target temperature KT₂. As a result, thetemperature within the substrate 62 is maintained at the second targetvalue KT₂ with even temperature distribution throughout the entire massof the substrate 62. In the following step S7, the heaters H in thefirst vacuum chamber 52 are feed-back controlled so that the temperaturein the first or operating heating chamber 28 is maintained at apredetermined third target temperature KT₃ which is lower than thesecond target temperature KT₂ by the predetermined decrement value ΔKT.Step S8 is then implemented to determine whether the time during whichthe temperature in the operating heating chamber 30 is maintained at thesecond target temperature KT₂ has reached a second holding time HT₂(=HT_(N) where N=2). Immediately after the counter N is incremented to“2”, a negative decision (NO) is obtained in step S8, and steps S6-S8are repeatedly implemented. The second holding time HT₂ corresponds tothe second soaking step for holding the temperature in the second oroperating heating chamber 28 at the second target temperature KT₂, andalso corresponds to a stand-by step for holding the temperature in thefirst or stand-by heating chamber 28 at the third target temperatureKT₃, to thereby prepare the stand-by heating chamber 28 for a thirdsoaking step.

[0061] In step S7 implemented after the counter N is incremented to “2”,the temperature in the first heating chamber 28 is controlled to thethird target temperature KT₃ which is lower by a value 2ΔKT than thefirst target temperature KT₁ which was established in the first soakingstep. Therefore, it takes a comparatively long time for the temperaturein the first heating chamber 28 to be lowered from the first targettemperature KT₁ down to the third target temperature KT₂. To lower thetemperature to the third target value KT₃ in a shorter time, cooling airis initially delivered from the cooling tubes 58 in the first vacuumchamber 52 to lower the temperature in the heating chamber 28 to a levellower than the third target temperature KT₃, and then the appropriateheaters H are feed-back controlled to raise the temperature to the thirdtarget temperature KT₃. Thus, the stand-by step performed in step S7 tocontrol the temperature in the stand-by heating chamber 28 consists ofan initial cooling step to lower the temperature to a level lower thanthe target value KT₃ by a suitable value, and a subsequent temperatureraising and holding step to hold the temperature at the third targetvalue KT³.

[0062] When an affirmative decision (YES) is obtained in step S8 duringrepeated implementation of steps S6-S8, the control flow goes to step S9(feeding step) in which the intermediate shutter device 24 is operatedto move the shutter member 40 to its upper open position, and theendless belt 18 is operated to feed the substrate 62 from the secondheating chamber 30 into the first heating chamber 28 which has been thestand-by heating chamber and will function as the operating heatingchamber. The shutter member 40 is then moved to its lower closedposition. This point of time is indicated at t4 in FIG. 8.

[0063] Then, the control flow goes to step S10 to determine whether thecontent of the counter N is equal to “6”. Steps S5-S10 are repeatedlyimplemented until the affirmative decision (YES) is obtained in stepS10, that is, until the temperature in the operating heating chamber ismaintained at a predetermined sixth target value KT₆ for a sixth holdingtime HT₆. Thus, the substrate 62 is moved heat-treated alternately inthe first and second heating chambers 28, 30 in an initial portion ofthe cooling period, which initial portion consists of the first throughsixth soaking steps (temperature equalizing steps) for establishing evendistribution of the temperature throughout the entire mass of thesubstrate 62. In these six soaking steps, the substrate 62 issequentially held at the respective target temperature values KT₁, KT₂,KT₃, KT₄, KT₅ and KT₆ for the respective holding times HT₁, HT₂, HT₃,HT₄, HT₅ and HT₆. The temperature values KT₁, KT₂, KT₃, KT₄, KT₅ and KT₆decrease in the predetermined increment ΔKT. In this respect, it isnoted that the temperature distribution of the substrate 62 has acomparatively large influence on the strain and melting state of theglass component in the substrate in the initial portion of the coolingperiod. When the affirmative decision (YES) is obtained in step S10 at apoint of time t8 indicated in FIG. 8, the control flow goes to step S11to turn off the heaters H.

[0064] Then, the control flow goes to step S12 to determine whether thecooling of the substrate 62 is completed. This determination is effectedon the basis of the temperature in the second heating chamber 30 inwhich the substrate 62 is accommodated at that time. The substrate 62 iskept in the second heating chamber 30 until an affirmative decision(YES) is obtained in step S12. When the affirmative decision is obtainedin step S12, step S13 is implemented to activate the outlet end shutterdevice 26 to move the shutter member 42 to its upper open position, andthe endless belt 18 is operated to move the substrate 62 out of thesecond heating chamber 30.

[0065] In the present embodiment described above, the substrate 62having a film-forming composition thereon is uniformly heated for thepredetermined first holding time HT₁ in the first soaking step of thecooling period, in the first heating chamber 28 which is held at thefirst target temperature KT₁. In the next feeding step, the substrate 62is fed into the second heating chamber 30 which is held at the secondtarget temperature KT₂ which is lower by the predetermined decrementvalue 66 KT than the first target temperature KT₁. In the next secondsoaking step, the substrate 62 is uniformly heat-treated at the secondtarget temperature KT₂ for the second holding time HT₂. Thus, thesubstrate 62 is alternately placed in the first and second heatingchambers 28, 30 and cooled at the temperatures which decrease in stepswith the predetermined decrement of ΔKT. Accordingly, the localvariation of the temperature within the substrate 62 is minimized. Wherethe substrate 62 is made of a glass material, the local dimensionalvariation of the substrate 62 and consequent misalignment of printedthick films are prevented, resulting in a considerable increase in theyield ratio of the product. In particular, the present invention iseffective for the substrate 62 made of soda-line glass. Where thesubstrate 62 has a multiplicity of thick-film resistors and rib wallsprinted on its surface, the glass component contained in these printedthick films and functioning as a bonding agent is uniformly or evenlymelted or fused since the variation in the temperature within thesubstrate 62 is minimized. Thus, the present arrangement is suitable forreducing a variation in the resistance of the thick-film resistors and avariation in the height dimension of the rib walls.

[0066] Further, the present embodiment makes it possible to use aninexpensive soda-lime glass for the substrate 62 for a large-sizedelectronic device, so that the cost of manufacture of the substrate 62can be made considerably lower than where a glass having a high strainpoint is used. In addition, the use of soda-lime glass is effective toprevent chipping of the substrate during handling thereof due to adifference of its hardness from those of the thick films formed thereon,and cracking of the substrate during firing thereof due to a differenceof its thermal expansion coefficient from those of the thick films.

[0067] It is also noted that the present shuttle type or two-chambertime firing apparatus 10 has only the first and second heating chambers28, 30 in which the substrate 62 is alternately placed for heattreatment thereof. This arrangement is advantageous in that thelongitudinal size of the apparatus 10 is significantly reduced. Aone-way feeding type firing apparatus has a length as large as about 50meters, for example, if this apparatus is adapted to slowly cool thesubstrate so as to prevent local dimensional variation of the substrateas in the present firing apparatus 10. On the other hand, the length ofthe present two-chamber type firing apparatus 10 is as small asone-fifth of the length of the continuous type firing apparatus.

[0068] In the present embodiment, the shutter devices 22, 24, 26 haverespective movable partition walls in the form of the shutter members38, 40, 42, which are opened only when the substrate 62 is moved intoand from the first and second heating chambers 28, 30 and which areclosed to secure gas tightness of the chambers 28, 30 while thesechambers are operating as the operating and stand-by heating chambers.Accordingly, the temperatures within these heating chambers 28, 30 areevenly distributed, making it possible to further reduce a variation inthe temperature within the substrate 62.

[0069] Other embodiments of this invention will be described. In theseembodiments, the same reference signs as used in the first embodimentwill be used to identify the functionally corresponding elements, andredundant description of these elements will not be provided.

[0070] Referring to the schematic view of FIG. 10, there is shown aone-way feeding type firing apparatus 100 adapted to feed the substrate62 continuously in one direction. The present firing apparatus 100 isprovided with a first belt conveyor 102, a second belt conveyor 104 anda third belt conveyor 106, which are operated independently of eachother and disposed in series. The substrate 62 is fed by these first,second and third belt conveyors 102, 104, 106 in the feeding directionthrough a tunnel type furnace 108.

[0071] The tunnel type furnace 108 has a heating zone 110 in which thesubstrate 62 is heated to the highest temperature, a gradually coolingzone 112 in which the substrate 62 is gradually or slowly cooled, and acooling zone 114 in which the substrate 62 is cooled by indirect aircooling to a level near the ambient or room temperature. The heatingzone 110 is provided with a temperature detector T_(CU) for detectingthe temperature in the heating zone 110, a heater H_(U) for raising thetemperature in the heating zone 110, and a first temperature controldevice T_(C) _(¹) for controlling the temperature in the heating zone110, according to a temperature raising pattern as indicated at A inFIG. 8.

[0072] The gradually cooling zone 112 includes shutter devices S₁-S₇ fordividing it into a plurality of heating chambers, more specifically,first through sixth heating chambers R₁-R₆. The gradually cooling zone112 further includes temperature detectors T_(C) _(¹) -T_(C) _(⁶) fordetecting the temperatures in the heating chambers R₁-R₆, heaters H_(D)_(¹) -H_(D) _(⁶) for heating the heating chambers R₁-R₆, and atemperature control device T_(C) _(²) for controlling the temperaturesin the heating chambers R₁-R₆ to respective first through sixth targettemperatures KT₁-KT₆.

[0073] In the present one-way feeding type firing apparatus 100constructed as described above, the temperature of the substrate 62 israised according to the temperature raising pattern as indicated at A inFIG. 8 while the substrate 62 is continuously fed by the first beltconveyor 102 through the heating zone 110. The second belt conveyor 104is operated under the control of a drive control device not shown, suchthat the substrate 62 which has been fed by the first belt conveyor 102is intermittently fed through the first through sixth heating chambersR₁-R₆ which are held at the respective first through sixth targettemperatures KT₁-KT₆, so that the temperature of the substrate 62 isgradually lowered to the sixth target temperature KT₆, with evendistribution of the temperature within the substrate 62 in each heatingchamber R. The substrate 62 is then fed from the sixth heating chamberR₆ onto the third belt conveyor 106.

[0074] As a result, the substrate 62 is gradually cooled as it is fed bythe second belt conveyor 104 through the gradually cooling zone 112,with the substrate 62 being held at the respective first through sixthtarget temperatures KT₁-KT₆ in the first through sixth heading chambersR₁-R₆, for the predetermined holding time HT in each heating chamber R.The substrate 62 is then cooled down to a level near the ambienttemperature while it is continuously fed through the cooling zone 114 bythe third belt conveyor 106. Thus, the substrate 62 is cooled with itstemperature being lowered initially in steps and then continuously at agiven rate, as indicated in FIG. 8.

[0075] In the present second embodiment, too, the substrate 62 having afilm-forming composition provided thereon is cooled, first in the firstsoaking step in the first heating chamber R₁ for the predeterminedholding time -HT while the temperature in the first heating chamber R₁is maintained at the first target temperature KT₁. In the next feedingstep, the substrate 62 is fed into the second heating chamber R₂ whichis held at the second target temperature KT₂ lower than the first targettemperature KT₁ by the predetermined decrement ΔKT. In the next secondtemperature holding step, the substrate 62 is held in the second heatingchamber R₂ for the holding time HT. Then, the substrate 62 is fed intothe third through sixth heating chambers R₃-R₆ whose temperaturesdecrease in steps with the predetermined increment ΔKT. Accordingly, thelocal variation of the temperature within the substrate 62 is minimized.Where the substrate 62 is made of a glass material, the localdimensional variation of the substrate 62 and consequent misalignment ofprinted thick films are prevented, resulting in a considerable increasein the yield ratio of the product. Where the substrate 62 has amultiplicity of thick-film resistors and rib walls printed on itssurface, the glass component contained in these printed thick films andfunctioning as a bonding agent is uniformly or evenly melted or fusedsince the variation in the temperature within the substrate 62 isminimized. Thus, the present arrangement is suitable for reducing avariation in the resistance of the thick-film resistors and a variationin the height dimension of the rib walls.

[0076] In the present embodiment, the substrate 62 is heat-treated foreven temperature distribution while it is intermittently fed in onedirection by the second belt conveyor 104 through the first throughsixth heating chambers R₁-R₆. Accordingly, the overall length of thepresent firing apparatus 100 can be made smaller than that of aconventional continuous feeding type firing apparatus which is adaptedto continuously feed the substrate so as to cool the temperature of thesubstrate according to a continuous temperature cooling pattern thatpermits the substrate to have an extremely reduced local temperaturevariation. Since the present apparatus 100 does not have a stand-byheating chamber as provided in the two-chamber type apparatus 10, theapparatus 100 provides an accordingly increased degree of heat treatingefficiency and is suitable for mass production of a product using thesubstrate 62.

[0077] Referring to the schematic view of FIG. 11, there is shownanother one-way feeding type firing apparatus 116 adapted to fire thesubstrate 62 in one direction. The present firing apparatus 116 includesa first conveyor device 118, a plurality of second conveyor devices 120a-120 f, and a third conveyor device 122, which are operatedindependently of each other and disposed in series to feed the substrate62 in the predetermined feeding direction through tunnel type furnaces124 a and 124 b.

[0078] Each of the tunnel type furnaces 124 has an inner wall which ismade of a refractory glass such as crystallized glass containingβ-spodumene. The furnace 124 has a pre-heating zone 126, a heating zone128, a gradually cooling zone 130, and a cooling zone 132. Thepre-heating zone 126 is provided for heating the substrate 62 to thehighest temperature (heat-treatment temperature) and removing or burningoff, in this heating step, a binder (resin) contained in the filmsprinted on the substrate 62. The heating zone 128 is for maintaining thesubstrate 62 at the highest temperature for a predetermined time. Thegradually cooling zone 130 is for gradually cooling the substrate 62,while the cooling zone 132 is for cooling the substrate 62 down to alevel near the ambient temperature.

[0079] The first conveyor device 118 is provided extending through thepre-heating zone 126 and the heating zone 128. This first conveyordevice 118 includes a motor 134 with a speed reducer disposed below thefurnace 124 a, a chain 136, a plurality of line shafts 138 a-138 edisposed in a line, and a plurality of miter gears 140 a-14-f eachdisposed between the appropriate adjacent ones of the line shafts 139. Arotary motion of the motor 134 is transmitted to the substrate 62 forcontinuously feeding the substrate 62 through divisions of the furnace124, which are arranged in the longitudinal direction of the furnace124.

[0080] FIGS. 12(a) and 12(b) show in enlargement end portions of thefirst conveyor device 118, and FIG. 12(c) shows a part of the secondconveyor device 120. As shown i FIGS. 12(a) and 12(b), the miter gear140 a of the first conveyor device 118 has a driving shaft 142 a whoseaxis extends in the longitudinal direction of the furnace 124, andfurther has a driven shaft 144 a whose axis extends in a directionperpendicular to the longitudinal direction of the furnace 124. Namely,the axis of the driven shaft 144 a is perpendicular to the plane of thesheet of FIG. 12(a). The rotary motion of the motor 134 is transmittedto the miter gear 140 a through the chain 136, so that the driving shaft142 a is rotated in the direction indicated by arrows in FIG. 12(a),whereby the line shaft 138 a connected to the driving shaft 142 athrough a coupling 146 is rotated in the same direction as the drivingshaft 142 a, while at the same time the driven shaft 144 a is rotated inthe direction indicated by arrows in FIG. 12(a). It is noted that theline shafts 138 b-138 e are connected to the respective miter gears 140b-140 e through respective couplings (not shown) similar to the coupling140.

[0081] The miter gear 140 f shown in FIG. 12(b) is disposed at the rightend of the first conveyor device 118 as shown in FIG. 11. The drivingshaft 142 f is connected at one end thereof to the line shaft 138 ethrough a one-way coupling 148. At the other end of the driving shaft142 f, there is provided a motor 150 with a speed reducer, which isconnected to that other end of the driving shaft 142 f through a one-waycoupling 152. These one-way couplings 148, 152 are adapted to transmitrotary motions of the line shaft 138 e and motor 150 only in thedirection indicated by arrows in FIG. 12(b). As described below, themotor 150 is operated intermittently at a higher speed than the lineshaft 138, in synchronization with the second conveyor device 120. Inthis arrangement, a rotary motion of the line shaft 138 e is transmittedto the driving shaft 142 f through the one-way coupling 148 when themotor 150 is at rest. When the motor 150 is in operation, the rotarymotion of the line shaft 138 e is transmitted to the driving shaft 142 fthrough the one-way coupling 152, with the one-way coupling 148 held ina slipping state.

[0082] Below the plurality of miter gears 140, there are disposed aplurality of rotary shafts 154 whose axes extend in parallel with eachother in the direction perpendicular to the longitudinal direction ofthe furnace 124. These rotary shafts 154 are arranged in thelongitudinal direction of the furnace 124. Thus, the first conveyordevice 118 is provided with multiple rotary shafts 154 arranged in thelongitudinal direction of the furnace 124 over the entire length. Therotary motion of the driven shaft 144 is transmitted to the rotaryshafts 154 through a chain (or timing belt) 156 so that the rotaryshafts 154 are rotated in the direction indicated by arrow. When themotor 134 is operated, the multiple rotary shafts 154 of the firstconveyor device 118 arranged in the longitudinal direction of thefurnace 124 are concurrently rotated at the same speed in the samedirection. When the motor 150 is operated, the rotary motion of themotor 150 is transmitted to the miter gear 140 f, and the miter gear 140f is substantially disconnected from the line shaft 138 f by the one-waycoupling 148, whereby the rotary shafts 154 disposed below the mitergear 140 f, that is, the rotary shafts 154 in a half area 128 b of theheating zone 128 on the side of the gradually cooling zone 130 arerotated at a higher speed that the other rotary shafts 154 of the firstconveyor device 118. In the present embodiment, the heating zone area128 b through which the substrate 62 is fed by the miter gear 140 f andwhich is adjacent to the gradually cooling zone 130 is an area in whichthe feeding speed of the substrate 62 is changed. Further, the motor 150and the one-way couplings 148, 150 function as a device for changing thefeeding speed of the substrate 62.

[0083] Each of the plurality of second conveyor devices 120 is equippedwith a motor 150 with a speed reducer, as shown in FIG. 12(c). Themotors 150 of the second conveyor devices 120 are operatedintermittently independently of each other. As in the first conveyordevice 118, a plurality of rotary shafts 154 are disposed below themotor 158 such that the axes of the rotary shafts 154 are perpendicularto the longitudinal direction of the furnace 124. These rotary shafts154 are connected to an output shaft 160 of the motor 158 through achain 156, and are rotated in the same direction by the motor 158. Thus,the plurality of second conveyor devices 120 use the independentlyoperated motors 158, in place of the miter gears 140 rotated by themotor 134 in the first conveyor device 118. As is apparent from FIG. 11,the third conveyor device 122 include the miter gears 140 the number ofwhich is smaller than that in the first conveyor device 118. Further,the arrangement of the third conveyor device 122 is reversed withrespect to the first conveyor device 118, in the longitudinal directionof the furnace 124. The third conveyor device 122 includes a motor 162with a speed reducer disposed below the furnace 124 b. A rotary motionof the motor 162 is transmitted to miter gears 140 g, 140 h, 140 i andline shafts 138 f, 138 g. The miter gear 140 g has the same constructionas the miter gear 140 f. A rotary motion of a motor 164 with a speedreducer is transmitted to the miter gear 140 g through a one-waycoupling, so that the rotary shafts 154 corresponding to the miter gear140 g are intermittently rotated at a higher speed than the other rotaryshafts 154, that is, at a speed synchronous with the second conveyordevices 120. Accordingly, a half area 132 a of the cooling zone 132through which the substrate 62 is fed by the miter gear 140 g and whichis adjacent to the gradually cooling zone 130 is an area in which thefeeding speed of the substrate 62 is changed. The motor 164 constitutesa part of a device for changing the feeding speed in the area 132 a.

[0084] Referring to FIG. 13, there is shown the furnace 124 inlongitudinal cross section taken at a widthwise middle portion thereof,with some longitudinal portions thereof being omitted. In the furnace124, there are disposed a plurality of rollers 166 made of alumina, forexample. As shown in the cross sectional views of FIGS. 14(a)-14(e)taken along lines a-a through e-e of FIG. 13, each roller 166 extendsthrough the furnace 124 in the width direction and is supported at itsopposite end portions by the furnace 124. While FIG. 14(a) is the crosssectional view taken along line a-a of FIG. 13, a cross sectional viewtaken along line a2-a2 of FIG. 13 is the same as FIG. 14(a). Thesubstrate 62 indicted above is placed on the rollers 166 in the furnace124. When the rollers 166 are rotated, the substrate 62 is fed in onedirection. The rollers 166 are connected to the rotary shafts 154coaxially therewith, for simultaneous rotation therewith. The rollers166 provided in the first and third conveyor devices 118, 122 in thepre-heating zone 126, heating zone 128 and cooling zone 132 are rotatedcontinuously to feed the substrate 62 continuously. On the other hand,the rollers 166 provided in the second conveyor devices 120 in thegradually cooling zone 130 are rotated intermittently to feed thesubstrate intermittently. In the present embodiment, therefore, thefirst and third conveyor devices 118, 122 constitute a continuouslyfeeding device, while the second conveyor devices 120 constitute anintermittently feeding device.

[0085] It will be understood from FIGS. 11, 13 and 14 that a pluralityof temperature detectors TC are provided in the pre-heating zone 126 todetect the temperatures in the pre-heating zone 126. Described morespecifically, three temperature detectors TC are disposed on each of theupper and lower sides of the furnace 124 in each of longitudinaldivisions of the pre-heating zone 126 which corresponds to the mitergears 140. The three temperature detectors TC are located at alongitudinally middle portion of each longitudinal division and arrangedin the width direction of the pre-heating zone 126. Further, a pluralityof heaters H are disposed on each of the upper and lower sides of thefurnace 124, for each of the longitudinal divisions of the pre-heatingzone 126. The heaters H are disposed in a matrix defined by four columnsparallel to the longitudinal direction and four rows parallel to thewidth direction. The output signals of the temperature detectors TC arefed to a control device 168, as shown in FIG. 13 by way of example inconnection with the set of upper temperature detectors TC and the set ofupper heaters H in the inlet end portion of the pre-heating zone 126.Based on the output signals of the temperature detectors TC, the controldevice 168 controls the heaters H so as to raise the temperature in thepre-heating zone 126 according to a temperature raising pattern asindicated at A in FIG. 8, while the substrate 62 is fed through thepre-heating zone 126. The pre-heating zone 126 is further provided withan intake pipe 170 at the inlet end of the furnace 124, and exhaustpipes 170 in the above-indicated longitudinal divisions except the inletend division in which the intake pipe 170 is provided. Each exhaust pipe170 is located at the upstream or leading end of the appropriatelongitudinal division as seen in the feeding direction of the substrate62. The intake pipe 170 is connected to an inlet conduit provided on theside surface of the furnace 124, for supplying air from an air source(not shown) into the furnace 124. The exhaust pipes 172 are connected toexhaust conduits 176 provided on the side surface of the furnace 124,for discharging the air from the furnace 124 while the air flows throughthe furnace 124. The intake pipe 170 and exhaust pipes 172 have nozzlessimilar to the nozzle 57 of the cooling tubes 58, in their portionslocated within the furnace 124. The nozzles of the exhaust pipes 172 maybe elongate holes extending in the longitudinal direction of the pipes,for improving the air exhausting efficiency.

[0086] In the heating zone 128, too, a plurality of temperaturedetectors TC are provided to detect the temperatures in the heating zone128. Namely, nine temperature detectors TC are disposed on each of theupper and lower sides of the furnace 124 in each of longitudinaldivisions of the heating zone 128, which divisions are similar to thoseof the pre-heating zone 126. These nine temperature detectors TC aredisposed in a matrix defined by three columns and three rows parallel tothe longitudinal and width directions of the furnace 124. Further,heaters H are disposed on the upper and lower surfaces and an upperportion of each of the opposite side surfaces of the furnace 124, ineach of the longitudinal divisions. The heaters H on the upper and lowersurfaces in each longitudinal division of the furnace 124 are disposedin a matrix defined by four columns and four rows parallel to thelongitudinal and width directions of the furnace 124. The heaters H oneach side surface of the furnace 124 consist of four heaters disposed ina row in the longitudinal direction. The temperature detectors TC andheaters H provided in the heating zone 128 are also connected to thecontrol device 168, so that the heaters H are controlled based on theoutput signals of the temperature detectors TC, so as to maintain thetemperature in the heating zone 128 as indicated at HT in FIG. 8, whilethe substrate 62 is fed through the heating zone 128.

[0087] The gradually cooling zone 130 includes shutter devices S₁-S₇ fordividing the zone 130 into six heating chambers R₁-R₆, temperaturedetectors TC for detecting the temperatures in the heating chambersR₁-R₆, and a set of heaters H for each of the heating chambers R₁-R₆. Asin the heating zone 128, the heaters H for each heating chamber R areprovided on the upper and lower surfaces and an upper portion of eachside surface of the furnace 124. The gradually cooling zone 130 furtherincludes upper and lower intake pipes 180 provided at the downstream ortrailing end of each heating chamber R as seen in the feeding directionof the substrate 62, for introducing cooling air into the heatingchamber R, and an exhaust pipe 182 provided at the upstream or leadingend of each heating chamber R as seen in the feeding direction, fordischarging the cooling air from an upper portion of the heatingchamber. The intake and exhaust pipes 180, 182 are connected to externalintake and exhaust conduits 184, 186, respectively, so that the coolingair is supplied from an air source into the heating chamber R throughthe intake pipe 180, and the air is discharged from the heating chamberR through an exhaust outlet 188. Like the intake and exhaust pipes 170,172 in the pre-heating zone 126, the intake and exhaust pipes 180, 182are made of alumina ceramics, and have a multiplicity of nozzles orholes. The temperature detectors TC and heaters H in the graduallycooling zone 130 are also connected to the control device 168, and theheaters H are controlled by the device 168 so that the temperatures inthe six heating chambers R are controlled to KT₁-KT₆ which decrease insteps as indicted in FIG. 8. Thus, the first heating chamber R₁corresponds to the first soaking step. However, the temperature in thefirst heating chamber R₁ may be controlled to KT₂ rather than KT₁.

[0088] Referring to the cross sectional view of FIG. 14(d) taken alongline d-d of FIG. 13, there is shown the shutter device S₂ by way ofexample. Each shutter device S includes a partition plate 190, shutterguides 192, 192, a shutter 194, three small jacks 196, and a motor 198.The partition plate 190 is made of a refractory glass similar to arefractory glass of the inner wall of the furnace 124. The shutter 194functions as a movable partition wall which is vertically moved betweenthe rollers 166. The small jacks 196 are disposed below the furnace 124and are driven by the motor 198 to vertically move the shutter 194. Thepartition plate 190 consists of two refractory glass plates which arefixed on an upper portion of the furnace 124 such that the tworefractory glass plates are spaced apart from each other by a smalldistance corresponding to the thickness of the shutter 194. Each shutterguide 192 is provided on the side surface of the furnace 124, adjacentto the lower surface of the partition plate 190. Like the partitionplate 190, the shutter guide 192 consists of two refractory glass plateswhich are spaced apart from each other by the small distancecorresponding to the thickness of the shutter 194. Thus, the partitionplate 190 has a groove formed to extend along the upper wall of thefurnace 124, while the shutter guides 192 guide grooves formed to extendalong the side walls of the furnace 124. These grooves have a dimensionin the longitudinal direction of the furnace 124, which is almost equalto the thickness of the shutter 194. In this arrangement, the shutter194 is vertically guided by the guide grooves of the shutter guides 192,and the upper end portion of the shutter 194 is received in the grooveof the partition plate 190 when the shutter 194 is placed in its upperclosed position, as indicated in FIG. 13 with respect to the shutterdevice S₁ by way of example. The two small jacks 196 are connected toeach other by a drive shaft 200 and are simultaneously rotated by themotor 198.

[0089] In the cooling zone 132, temperature detectors TC for detectingthe temperatures in the cooling zone are provided on the upper side ofthe furnace 124, in each of the longitudinal divisions of the coolingzone 132 as in the pre-heating and heating zones 126, 128. Thesetemperature detectors TC are located in the longitudinally and widthwisemiddle portions of the longitudinal division of the cooling zone 132.Further, three cooling jackets C are provided on each of the upper andlower sides of the furnace, in each longitudinal division of the coolingzone 132, such that the cooling jackets C extend in the direction ofwidth and are arranged in the longitudinal direction. The dimension ofeach jacket C in the direction of width of the cooling zone 132 issubstantially equal to the width of the furnace 124 as indicated in FIG.14(e). Cooling water supplied from a cooling water pipe 202 as shown inFIG. 11 is circulated through the cooling jackets C, and the rates offlow of the cooling water in the cooling jackets C in the individuallongitudinal divisions of the cooling zone 132 are controlled byrespective solenoid-operated valves. The temperature detectors TC andthe solenoid-operated valves for the cooling jackets C are alsoconnected to the control device 168, so that the valves are controlledby the control device 168 based on the output signals of the temperaturedetectors TC, to control the rates of flow of the cooling water throughthe cooling jackets C according to a temperature lowering pattern asindicated in FIG. 8, while the substrate 62 is fed through the coolingzone 132.

[0090] In the one-way feeding type firing apparatus 116 constructed asdescribed above, the temperature in the pre-heating zone 126 is raisedaccording to the temperature raising pattern as indicated at A in FIG. 8while the substrate 62 is continuously fed through the pre-heating zone126 by the first conveyor device 118, and the temperature in the heatingzone 128 is maintained at the highest firing temperature KT₁ while thesubstrate 62 is continuously fed through the heating zone 128 also bythe first conveyor device 118. The second conveyor devices 120 areintermittently or sequentially operated under the control of a drivecontrol device (not shown) to feed the substrate 62 received from thefirst conveyor device 118 into the first heating chamber R₁ whosetemperature is maintained at the first target value KT₁, and thesubstrate 62 is held in the first heating chamber R₁ so that thetemperature within the substrate 62 is evenly distributed in the firstsoaking step. Then, the substrate 62 is sequentially introduced into thesecond through sixth heating chambers R₂-R₆ so that the substrate 62 issubjected to the second through sixth soaking steps at the temperaturesKT₂-KT₆ which decrease in steps. Then, the substrate 62 is fed from thesixth heating chamber R₆ onto the third conveyor device 122.

[0091] Thus, the substrate 62 is gradually cooled with even distributionof the temperature throughout the entire mass of the substrate 62 whilethe substrate 62 is fed by >the second conveyor devices 120 through thegradually cooling zone 130 such that the substrate 62 is held for thepredetermined holding time in each of the first through sixth heatingchambers R₁-R₆ whose temperatures are held at the respective firstthrough sixth target values KT₁-KT₆. The substrate 62 is thencontinuously cooled down to a level near the ambient or room temperaturewhile the substrate 62 is continuously fed by the third conveyor device122 through the cooling zone 132. As described above, the substrate 62whose temperature is raised to and held at the highest firingtemperature KT₁ is cooled relatively slowly in the initial portion ofthe cooling period with stepwise lowering of the temperature, andrelatively rapidly in the following portion of the cooling period withcontinuously lowering of the temperature at a suitable rate. It will beunderstood from the foregoing explanation of the present embodiment thatthe pre-heating zone 126, heating zone 128 and cooling zone 132constitute a continuous heat treatment area in which the substrate 62 isheat-treated while it is continuously fed, and that the first and thirdconveyor devices 118, 122 constitute a continuously feeding device forfeeding the substrate 62 continuously in one direction.

[0092] In the present third embodiment, too, the substrate 62 having afilm-forming composition thereon is heat-treated for even distributionof the temperature in the first soaking step while the substrate 62 isheld at the predetermined first target temperature KT₁ in the firstheating chamber R₁ for the predetermined holding time HT. Then, thesubstrate 62 is fed in the feeding step into the second heating chamberR₂ whose temperature is maintained at the second target value KT₂ whichis lower than the first target value KT₁ by the predetermined decrementΔKT. In this second heating chamber R₂, the substrate 62 is subjected tothe second soaking step for the predetermined time so that thetemperature within the substrate 62 is evenly distributed throughout theentire mass of the substrate. The substrate 62 is then sequentiallysubjected to soaking steps in the third through sixth heating chambersR₃-R₆ at the temperatures KT₃-KT₆ which decrease in steps with thedecrement value ΔKT, so that the temperature throughout the substrate 62is evenly distributed in each heating chamber R while it is held thereinfor the predetermined time. Thus, the present embodiment is alsoeffective to minimize a local variation in the temperature within thesubstrate 62. Where the substrate 62 is made of a glass material, thelocal dimensional variation of the substrate 62 and consequentmisalignment of printed thick films are prevented, resulting in aconsiderable increase in the yield ratio of the product. Where thesubstrate 62 has a multiplicity of thick-film resistors and rib wallsprinted on its surface, the glass component contained in these printedthick films and functioning as a bonding agent is uniformly or evenlymelted or fused since the variation in the temperature within thesubstrate 62 is minimized. Thus, the present arrangement is suitable forreducing a variation in the resistance of the thick-film resistors and avariation in the height dimension of the rib walls.

[0093] In the present embodiment, the substrate 62 is subjected to thesoaking steps in the first through sixth heating chambers R₁-R₆ whilethe substrate 62 is intermittently fed in one direction by the secondconveyor devices 120. Accordingly, the overall length of the presentfiring apparatus 116 can be made smaller than that of a conventionalcontinuous feeding type firing apparatus which is adapted tocontinuously feed the substrate so as to cool the temperature of thesubstrate according to a continuous temperature cooling pattern thatpermits the substrate to have an extremely reduced local temperaturevariation. Since the present apparatus 116 does not have a stand-byheating chamber as provided in the two-chamber type apparatus 10, theapparatus 116 provides an accordingly increased degree of heat treatingefficiency and is suitable for mass production of a product using thesubstrate 62.

[0094] In the present embodiment, the first conveyor device 118, secondconveyor devices 120 and third conveyor device 122 include the rollers166 which are disposed in the tunnel type furnace 124 and whose axes areparallel to each other and perpendicular to the longitudinal directionof the furnace 124. These rollers 166 are rotated about their axes tofeed the substrate 62 in one direction through the furnace 124. In thisarrangement wherein the substrate 62 is supported and fed in the feedingdirection by the rotating rollers 166, the films formed on the substrate62 are less likely to be adversely influenced by dust which may beconsiderably scattered in the furnace 124 where a conveyor belt is usedto feed the substrate in the furnace. Namely, the feeding of thesubstrate 62 by the rotating rollers 166 is less likely to deterioratethe function of the films on the substrate 62 due to dust in the furnace124 during the heat treatment therein. Thus, the present thirdembodiment uses the rollers 166 in place of the belt conveyor 102 usedin the second embodiment which includes the endless belt 18 made from amesh of refractory metal. In other words, the present firing apparatus116 employs a so-called roller hearth kiln.

[0095] The shutter devices S used in the present embodiment include theshutters 194 which are vertically movable between the appropriateadjacent ones of the rollers 166, so as to divide the furnace 124 intothe thermally insulated first through sixth heating chambers R₁-R₆. Theshutters 194 are vertically movable without an interference with therollers 166, so that the individual shutter devices S are completelyindependent of each other, permitting complete thermal insulation of theheating chambers R, and assuring improved uniformity of temperature ineach heating chamber R and accordingly reduced local variation of thetemperature within the substrate 62.

[0096] The rollers 166 of the conveyor devices 118, 120, 122 are formedof alumina ceramics, and are less likely to be worn, rusted, damaged ordeteriorated due to contact with the substrate 62 and heating in thefurnace 124, assuring an reduced amount of dust produced in the furnace124 and accordingly enhanced quality of the fired substrate 62.

[0097] Further, the inner wall of the tunnel type furnace 124 and thepartition plates 190, shutter guides 192 and shutters 194 of the shutterdevices S are formed of a ceramic material, more specifically, arefractory glass such as β-spodumene crystallized glass. Thus, therollers 166 and the inner wall surfaces of the furnace 124 including theinner surfaces of the heating chambers R₁-R₆ are all made of the ceramicmaterial, and are less likely to be worn, rusted, damaged ordeteriorated due to heating, assuring a further reduced amount of dustproduced in the furnace 124.

[0098] The firing apparatus 116 of the present embodiment has thepre-heating zone 126, heating zone 128 and cooling zone 132 whichconstitute the continuous heat treatment area which is adjacent to thegradually cooling zone 130 and in which the substrate 62 is heat-treatedwhile it is continuously fed. Further, the firing apparatus 116 includesthe first and third conveyor devices 118, 122 which constitute thecontinuously feeding device for feeding the substrate 62 continuously inone direction at a suitable speed by rotation of the rollers 166. Thecontinuous heat treatment area includes the areas 128 b, 132 a adjacentto the gradually cooling zone 130, and the first and third conveyordevices 118, 122 includes the motors 150, 164 and one-way couplings 148,152 which constitute the feeding speed changing device for controllingthe feeding speeds of the substrate 62 in the above areas 128 b, 132 aso that these feeding speeds are almost equal to the feeding speed inthe gradually cooling zone 130.

[0099] In the above arrangement, the substrate 62 is intermittently fedthrough the gradually cooling zone 130, and is continuously fed at thesuitable speed through the pre-heating zone 126, heating zone 128 andcooling zone 130. In the above-indicated areas 128 b and 132 a of thecontinuous heat treatment area which are adjacent to the graduallycooling zone 130, however, the rotating speed of the rollers 166 and thefeeding speed of the substrate 62 are raised to that of the graduallycooling zone 130, so that the substrate 62 may be smoothly andrelatively rapidly fed from the area 128 b into the gradually coolingzone 130, and from this zone 130 into the area 132 a, so that the timesrequired for the substrate 62 to move between the area 128 b and thezone 130 and between the zone 130 and the area 132 a are shortened,making it possible to reduce the local variation of the temperaturewithin the substrate 62 due to a difference in the temperatures betweenthe zone 130 and the areas 128 b, 132 a. Further, the feeding speedchanging device is effective to reduce an amount of sliding movementbetween the rollers 166 and the substrate 62 due to the difference inthe feeding speed between the areas 128 b, 132 a and the zone 130 (moreprecisely, first and sixth heating chambers R₁ and R₆), whereby theamount of dust produced in the furnace 124 is accordingly reduced. It isalso noted that the shutters 194 of the shutter devices S are placed intheir open position for a time as short as possible, so as to minimize adeviation of the temperature in each heating chamber R from the targetvalue KT and an uneven temperature distribution within each heatingchamber R.

[0100] While the presently preferred embodiments of this invention havebeen described above by reference to the accompanying drawings, it is tobe understood that the invention is not limited to the details of theillustrated embodiments, but may be otherwise embodied.

[0101] While the nine pairs of heaters H are provided in each of thefirst and second vacuum chambers 52, 54 corresponding to the first andsecond heating chambers 28, 30, a pair of relatively large heaters maybe provided in each vacuum chamber 52, 54, on the upper and lower sidesof the heating chamber 28, 30. In this case, the amount of heatgenerated by the heaters may be made larger in the peripheral portion ofthe heaters by increasing the density of heater windings in theperipheral portion, for example.

[0102] In the second and third embodiments of FIGS. 10 and 11, the sizeof each of the first through sixth heating chambers R₁-R₆ in the feedingdirection may be increased so that the substrate 62 can be continuouslyfed by the second belt conveyor 104 or the second conveyor devices 120.

[0103] In the inlet, intermediate and outlet shutter devices 22, 24, 26of the embodiment of FIG. 1 and the shutter devices S₁-S₇ of theembodiments of FIGS. 10 and 11, the shutter members 38., 40, 42 and theshutters 194 are vertically movable. However, the shutter devices mayuse stationary partition members which permanently close the heatingchambers so as to permit feeding movements of the substrate 62 andassure substantial thermal insulation between the heating chambers forindependent control of the temperatures within the individual heatingchambers. In this case, the stationary partition members may be disposedsuch that their lower ends are spaced from the upper surface of theendless belt or from the rollers 166, by a distance slightly larger thanthe thickness of the substrate 62, so that the substrate 62 can passthrough gaps between the belt or rollers and the stationary partitionmembers of the shutter devices.

[0104] In the illustrated embodiments, the first, second and thirdtarget temperatures KT₁, KT₂ and KT₃ are determined to be close to thetransition or strain point of a glass contained in the substrate 62 orfilms printed thereon, so that the temperature of the substrate 62changes through the transition or strain point while the temperaturewithin the substrate 62 is evenly distributed throughout the entire massof the substrate. Where the films are bonded to the substrate 62 bymelting or sintering of a metallic or inorganic material, the first,second and third target temperatures KT₁, KT₂ and KT₃ are determined tobe close to the melting or sintering point of the metallic or inorganicmaterial, so that the temperature of the substrate 62 changes throughthe melting or sintering point while the temperature within thesubstrate 62 is evenly distributed throughout the entire mass of thesubstrate.

[0105] In the first embodiment of FIG. 1, the cooling period or processincludes the soaking steps wherein the second soaking step is effectedat the second target temperature KT₂ which is lower than the firsttarget temperature KT₂ by the predetermined decrement ΔKT. Where thetemperature raising process in which the temperature is raised isimportant, soaking steps may be provided also in the temperature raisingprocess. In this case, the second target temperature KT₂ is higher thanthe first target temperature KT₁ by the predetermined increment ΔKT.

[0106] In the third embodiment of FIG. 11, the miter gears 140 of thefirst and third conveyor device 118, 122 are driven by the single motor134 or 162 through the line shafts 138. However, the miter gears 140 maybe replaced by respective motors similar to the motor 158 provided ineach second conveyor devices 120.

[0107] In the embodiment of FIG. 11, the feeding speed changing deviceincluding the motors 150, 164 is provided for changing the feeding speedof the substrate 62 in the areas 128 b, 132 a adjacent to the graduallycooling zone 130. However, the feeding speed changing device may beeliminated. In this case, the feeding speed in the area 128 b, 132 a isthe same as that in the other area of the first or third conveyor device118, 122.

[0108] Although the rollers 166 in the embodiment of FIG. 11 are made ofalumina ceramic, they may be made of other ceramics such as mullite orspodumene. Further, the rollers 166 may be made of stainless steel(e.g., SUS310) or other metallic material having a sufficiently highheat resistance, since the rollers 166 are not subject to frictionwithin the furnace.

[0109] It is to be understood that the prevent invention may be embodiedwith various other changes, modifications and improvements, which mayoccur to those skilled in the art, without departing from the spirit andscope of the invention defined in the following claims:

What is claimed is:
 1. A firing process of uniformly heat-treating a substrate having a film-forming composition thereon, comprising the steps of: a first soaking step of holding, for a predetermined first time, said substrate in a first heating chamber whose temperature is maintained at a predetermined first value, so that the temperature within said substrate is held at said first value evenly throughout an entire mass of said substrate; a feeding step of feeding said substrate subjected to said first soaking step, to a second heating chamber whose temperature is maintained at a predetermined second value which is different from said predetermined first value by a predetermined difference; and a second soaking step of holding said substrate in said second heating chamber for a predetermined second time, so that the temperature within said substrate is held at said second value evenly throughout the entire mass of said substrate.
 2. A firing process according to claim 1 , further comprising a first stand-by step which is implemented concurrently with said first soaking step, to adjust the temperature in said second heating chamber to said predetermined second value so that said second soaking step is implemented in said second heating chamber, and further comprising a second stand-by step which is implemented concurrently with said second soaking step, to adjust the temperature in said first heating chamber to a predetermined third value which is different from said predetermined second value by a predetermined difference, so that a third soaking step is implemented in said first heating chamber.
 3. A firing process according to claim 2 , wherein said first stand-by step comprises a cooling step of lowering the temperature in said second heating chamber to a value lower than said predetermined second value by a predetermined amount, and a temperature raising and holding step of effecting feed-back control to raise the temperature in said second heating chamber to said predetermined second value and maintain said second value, and wherein said second stand-by step comprises a cooling step of lowering the temperature in said first heating chamber to a value lower than said predetermined third value by a predetermined amount, and a temperature raising and holding step of effecting feed-back control to raise the temperature in said first heating chamber to said predetermined third value and maintain said third value.
 4. A firing apparatus for uniformly heat-treating a substrate having a film-forming composition thereon, comprising: shutter devices which partially define a first and a second heating chamber such that said first and second heating chambers are thermally insulated from each other; a heating device for controlling temperatures in said first and second heating chambers, independently of each other; a feeding device for feeding said substrate into said first and second heating chambers alternately, so that said substrate is heat-treated in said first and second chambers alternately; and a control device for controlling said heating device to maintain the temperature in said first heating chamber at a predetermined first value while said substrate is heat-treated in said first heating chamber, and adjusting the temperature in said second heating chamber to a predetermined second value different from said predetermined first value by a predetermined difference while said substrate is heat-treated in said first heating chamber, said control device adjusting the temperature in said first heating chamber to a predetermined third value different from said predetermined second value by a predetermined difference while said substrate is heat-treated in said second heating chamber, so that said substrate is then heat-treated in said first heating chamber at said third value.
 5. A firing apparatus according to claim 4 , further comprising a cooling device for lowering the temperatures in said first and second heating chambers.
 6. A firing apparatus according to claim 5 , wherein said cooling device comprises cooling tubes for delivering cooling air into said first and second heating chambers.
 7. A firing apparatus according to claim 4 , wherein each of said shutter devices includes a shutter member movable between an open position and a closed position for thermal insulation of said first and second heating chambers, said movable shutter member being placed in said open position when said substrate is fed by said feeding device into or from one of said first and second heating chambers, and in said closed position while said substrate is heat-treated in one of said first and second heating chambers and while the temperature in the other of said first and second heating chambers is adjusted.
 8. A firing apparatus for uniformly heat-treating a substrate having a film-forming composition thereon, comprising: shutter devices which partially define at least two heating chambers including a first and a second heating chamber such that said at least two heating chambers are thermally insulated from each other; a heating device for controlling temperatures in said first and second heating chambers independently of each other; a temperature control device for controlling said heating device to maintain the temperature in said first heating chamber at a predetermined first value uniformly throughout said first heating chamber, and to maintain the temperature in said second heating chamber at a predetermined second value uniformly throughout said second heating chamber, said second value being different from said first value by a predetermined difference; and a feeding device for feeding said substrate in one direction, first into said first heating chamber for heat-treating said substrate at said predetermined first value for a predetermined first time, and then into said second heating chamber for heat-treating said substrate at said predetermined second value for a predetermined second time, said feeding device further feeding said substrate from said second heating chamber after heat treatment thereof in said second heating chamber.
 9. A firing apparatus according to claim 8 , wherein each of said shutter devices includes a shutter member movable between an open position and a closed position for thermal insulation of said first and second heating chambers, said movable shutter member being placed in said open position when said substrate is fed by said feeding device into or from one of said first and second heating chambers, and in said closed position while said substrate is heat-treated in said first and second heating chambers.
 10. A firing apparatus according to claim 8 , wherein said feeding device comprises a plurality of rollers whose axes of rotation are parallel to each other and perpendicular to said one direction and which are arranged in said one direction to support said substrate, said rollers being rotated to feed said substrate in said one direction.
 11. A firing apparatus according to claim 10 , wherein one of said shutter devices includes a shutter which is movable in a vertical direction perpendicular to said one direction, between an open position and a closed position, through a gap between adjacent ones of said plurality of rollers, said shutter placed in said closed position separating said first and second heating chambers from each other with thermal insulation therebetween.
 12. A firing apparatus according to claim 10 , wherein each of said plurality of rollers is made of a ceramic material.
 13. A firing apparatus according to claim 12 , wherein each of said at least two heating chambers has an inner wall surface made of a ceramic material, and said shutter of said each shutter device is made of a ceramic material.
 14. A firing apparatus according to claim 10 , wherein said feeding device includes an intermittently feeding device for intermittently feeding said substrate by rotation of said plurality of rollers through said at least two heating chambers, and a continuously feeding device for continuously feeding said substrate by rotation of said plurality of rollers at a predetermined feeding speed through a continuous heat treatment zone which includes an area adjacent to said at least two heating chambers, said continuously feeding device including a feeding speed changing device for changing rotating speeds of said rollers in said area so that a feeding speed of said substrate in said area is almost equal to the feeding speed by said intermittently feeding device. 